The present invention relates to a method of soldering a semiconductor optical device to an object such as a printed circuit board, particularly to a method of soldering a semiconductor optical device including a resin-made optical lens by a reflow soldering process using a lead-free solder, and to a semiconductor optical device for use in the method.
Light emitting diodes (LEDs) include bullet-type ones, chip-type ones and the like, and the bullet-type light emitting diodes generally have a structure in which a light emitting element and conductive members such as a cathode lead, an anode lead, a light emitting semiconductor chip and a lead thin wire or wires are encapsulated with a light transmitting resin. The chip-type light emitting diodes generally have a structure in which a pair of lead frames are extended from the inner bottom of a top-open box-like light emitting element casing to the exterior of the light emitting element casing, whereas a light emitting semiconductor chip, lead thin wires and the like are contained in the inside of the light emitting element casing, these components are connected, and the inside of the casing is encapsulated with a light transmitting resin. These light emitting diodes may each be provided on the encapsulating material with an optical lens for regulating the propagation direction of the light emitted, according to the use purpose thereof.
In addition, an image forming light receiving device unit used as a small camera equipped in a cellular phone or the like is provided with a condensing optical lens on the optical path of the light incident on a semiconductor chip serving as a light receiving device.
Since the semiconductor optical devices such as the light emitting diodes and the image forming light receiving device units including the resin-made optical lens are generally connected to a circuit board or the like by soldering, the resin used as the material of the optical lens must have thermal resistance against the heat received at the time of soldering to the circuit board or the like. Therefore, an epoxy resin has hitherto been used as the material of the optical lens.
On the other hand, in recent years, due to environmental concern, conversion from the conventional lead-tin alloy solders to lead-free solders has been under way. The lead-free solders are higher in melting temperature than the conventional lead-tin alloy solders, and the encapsulating resins hitherto used lack thermal resistance at the higher temperatures.
Particularly, in the reflow soldering process generally used as a method of soldering a semiconductor optical device to a circuit board, the thermal resistance temperature required of the material for the optical lens has been about 230° C. in the cases of the conventional lead-tin alloy solders, but is as high as about 250° C. or above in the cases of the lead-free solders. However, the conventional epoxy resin does not have thermal resistance to the higher temperatures, so that an attempt to conduct the soldering by the reflow soldering process using a lead-free solder would result in that the lens is distorted or deteriorated and that the lens would be incapable of functioning as a lens. Therefore, the reflow soldering process using a lead-free solder is not applicable to the soldering of the semiconductor optical devices, and it may be necessary to individually solder the semiconductor optical devices. The individual soldering leads to a lowering in productivity.
Although optical lenses include glass lenses, formation of minute lenses for the semiconductor optical devices from glass needs extremely intricate steps and, therefore, is impractical. It is applicable to mount the optical lenses to the semiconductor optical devices after the reflow soldering. However, the semiconductor optical devices are mounted to the circuit board at diverse positions. Therefore, it is difficult to automate such the mounting process, and the process steps would eventually be intricate.
Therefore, there has been a request for a reflow soldering process by which a semiconductor optical device including a resin-made optical lens can be soldered by a lead-free solder.